As is known in the art, EMI (electromagnetic interference) filters include capacitive and inductive components to achieve acceptable signal characteristics. As is also known, filter capacitance can result in a leading power factor, which is highly undesirable in some applications. Thus, there are tradeoffs in EMI filtering and power factor characteristics. In conventional applications, relatively large inductors are used to minimize the effects of the capacitance. However, large inductors are large, heavy and costly.
FIG. 1 shows an exemplary prior art EMI filter for a three-phase power system 10. First, second, and third phases A, B, C are input at respective terminals. Common mode inductors L1-3, which are relatively small, are coupled to each terminal. Each phase is filtered using inductive and capacitive elements. For example, the first phase A is filtered by inductor L4 and capacitor C1. The second phase B is filtered by inductor L5 and capacitor C2, and the third phase C is filtered by inductor L6 and capacitor C3. In the illustrated embodiment, the filter inductors are 50 micro henries, which are quite large, to balance out the 1 micro farad capacitance of the filter capacitors C1-3. In general, the large inductors are used to ‘hide’ the capacitance of the filter capacitance.